1. Field of the Invention:
The present invention relates to a free-cutting steel having a high fatigue strength and outstanding machinability, which is suitable for use as mechanical structural parts such as crank-shafts, connecting rods, and axle shafts of automotive engines. 2. Description of the Prior Art:
Heretofore, mechanical structural parts such as crank-shafts of automotive engines have usually been made of structural carbon steel, such as S50C, or a steel containing such elements as S and Pb which improve machinability, by hot forging, hardening (quenching), and tempering. They are required to have a high fatigue strength because they are subject to damage resulting from fatigue failure.
One possible way to improve the fatigue strength of steels is to increase the hardness of steels. The increased hardness, however, decreases the machinability of steels. Machinability can be improved by the addition of such elements as S and Pb; but they lead to notches which lower the fatigue strength. Thus, fatigue strength machinability are mutually contradictory characteristics. The present invention was completed to address this problem.